Aerobic Capacity. The core of the Firstbeat approach is to estimate V̇O2max, both as a percentage and an absolute. Like other approaches, the Firstbeat estimation uses heart rate, but Firstbeat adds in Heart Rate Variability to detect breathing patterns that provide a better estimation.

Training Effect. The Firstbeat Training Effect (TE) is intended to measure the effectiveness of training stress. The TE is biased towards continuous high intensity training (AKA Tempo runs), with lower TE scores given for interval training. Even lower scores are given for longer runs that are at a lower intensity, so TE does not reflect overall training stress. I have had a higher Training Effect values from a 3 mile easy run than a 100 mile race! Training Effect is based Activity Class, which is a representation of how much exercise is routinely performed. (Suunto call this "Peak Training Effect" or PTE.)

EPOC. The Training Effect is based on estimation of EPOC (Excess Post-exercise Oxygen Consumption). This EPOC is how much extra oxygen the body requires while recovering from strenuous exercise. Originally it was thought that EPOC was the repaying the so-called "oxygen debt" of high intensity exercise due to the buildup of lactate. However, it seems that the actual mechanism is likely to be rather more complex. The use of Training Effect does not require an understanding of EPOC, but the Suunto website does expose the EPOC estimate for those that are interested.

Activity Class. The impact of any given belt of exercise will depend on your fitness level. It's reasonably obvious that a 5 mile run may be overwhelming for some people, but only an easy run for others. The Training Effect model takes this into account by classifying athletes by their Activity Class. Athletes are mostly classified by the number of hours of training they do each week. Generally, the Garmin watches automatically estimate Activity Class where the Suunto watches have it set manually. Obviously the automatic setting is much easier, but you have to wear the watch with a heart rate monitor for all workouts, or of the estimates will be invalid. It's also not possible to tweak the Activity Class when it set automatically.

Recovery Time. At the end of each run the Recovery Time suggests how long before the next hard training session. The Recovery Time takes into account multiple training sessions, reflecting cumulative fatigue. Unlike Training Effect, Recovery Time seems to reflect overall training stress. However, I've found no details of the underlying model used to estimate Recovery Time, nor have I found any validation.

Recovery State. The Recovery State is displayed a few minutes into a run and indicates how well recovered you are. Like Recovery Time, there is no indication of the underlying model or science behind this. I've never had any other message than "good recovery state", even the day after an ultra.

The table below shows which devices support the particular Firstbeat functionality.

Watch

Calorie Burn

Training Effect (Auto AC)

Training Effect (Manual AC)

Recovery Time

Recovery Check

Garmin 620

X

X

X

X

Fenix 2

X

X

X

X

Garmin 910XT

X

X

Garmin 310XT

X

Garmin 610

X

X

Ambit2

X

X

X

Examples

Examples of EPOC and PTE

At the 2014 Badwater 135 the Training Effect was 2.6, with the peak EPOC occurring after 9 hours.

At the 2014 Keys 100 the Training Effect was 2.5, with the peak EPOC occurring within the first hour. The other 18 hours had no impact on EPOC or the Training Effect.

This 30 minute easy run has a gradually rising EPOC that results in a Training Effect of 2.4, slightly less than a 100 mile race.

This brisk 3 miles run resulted in a similar EPOC curve, but the Training Effect was 3.1, quite a bit higher than the ultras. (The heart rate is in white.)

This shows how the Training Effect in orange increased during the brisk 3 mile run.

A High Intensity Interval Training run resulted in a Training Effect of 3.2. You can see the HIIT section by the elevated Heart Rate in Orange and the sharp rise in EPOC in white.